Selective Catalytic Reduction (SCR) systems provide a method for removing nitrogen oxide (NOx) emissions from fossil fuel powered systems. During SCR, a catalyst facilitates a reaction between a reductant, such as urea, and NOx to produce water and nitrogen gas, thereby removing NOx from the exhaust gas. Generally, the reductant is mixed with the exhaust upstream of the SCR catalyst.
Reductant dosing systems may be used to introduce the reductant, such as urea, into the exhaust stream. For example, urea may be mixed with water and stored in a tank. As the power system operates and produces exhaust, the urea mixture is pumped from the tank and intermittently sprayed into the exhaust stream via a nozzle.
A challenge to urea dosing systems relates to the relatively high ambient temperature at which urea mixtures may freezes (i.e. about −11° C.). As urea freezes, it expands in the dosing system and can cause damage to intricate components such as injector nozzles. Additionally, when exposed to elevated temperatures, urea can decompose and form a deposit that can clog dosing components.
U.S. Published Patent Application No. 2007/0163238 (hereinafter the '238 application), by Gerlach, discloses a method of purging a dosing system of urea. The '238 application discloses using pressurized air to purge urea from a metering valve and reagent pump when the engine is shutdown and a threat of freezing exists.
While the method disclosed in the '238 application may address the issue of urea freezing in a dosing system, it may fail to address the issue of urea decomposing at elevated temperatures while the engine is running. Such as the potential for urea that remains in a nozzle between injections to decompose and form a deposit.
The present disclosure is directed to overcoming one or more of the shortcomings in the existing technology.